Battens, or purlins, are long, thin, and flat pieces of building material that are attached to the roof trusses to which the roofing material is fixed.
In a wooden framed structure, the (often wooden) battens are traditionally square or rectangular in cross-section. In a steel framed structure, traditionally the (typically metal) battens each have the shape of a generally U-shaped channel with outwardly extending flanges, wherein each batten is installed inverted (with the channel of the “U” facing downwardly), and with the flanges screwed or otherwise attached to the roof trusses. The roofing material may then be affixed to the tops of the battens (the generally flat bases of the “U” shape). These battens are often referred to as “top hats”. Top hats may be of different heights and widths, and the ratio between height and width may vary, but they are generally symmetrical.
Both top hats and wooden battens are used with both roof tiles and sheet roofing such as corrugated metal roofs. Sheet roofing is generally much lighter than tiles, and its span is much greater than a tile. A tile generally is between 300 mm and 450 mm long, and each row of tiles must be supported by a batten, so the battens are spaced 300 mm to 450 mm apart. A piece of sheet roofing can be of almost any length, and the battens may be spaced at 1200 mm and still provide a strong base for the roof. As a result, many less battens are required to make a roof covered with sheet material than tiles. Therefore tiled roofs are much more expensive than roofs of sheet material, not only from the cost of tiles, but the cost of battens and the time taken to install so many more battens.
Most tiles have a lower lip which hooks over the batten and then the tile is nailed to the batten, either through the lip or from the top.
Wooden battens, due to the thickness of the wood, are not very strong and cannot hold the weight of a roof installer. Working at height to install a roof means that there is always the risk of falling. Anything that can be done to minimise the spaces through which a roof installer can fall is desirable.
A steel batten is much stronger than a wood batten and can help support the weight of a worker. (Where this document refers to steel battens, it should be understood that while steel is preferred, other metals or appropriate plastics might be used instead.) When steel battens are installed at a spacing of between 300 and 450 mm, if a worker were to slip, the battens would form a safety net and prevent the worker from falling to the ground.
Wooden battens can warp and have weak spots. Steel battens do not warp or suffer from weak spots. However, the strength of a steel batten can be affected by the shape of the cross section, how it is installed, and the thickness of the steel used. Steel battens or top hats may be overlapped at joins due to their shape, providing extra strength. Wooden battens cannot be overlapped as the roof would then have ridges.
Also, traditional steel battens have not been used with timber trusses and tiled roofs due to higher costs and changes in work practices being required.
However, wooden battens can be attached to the roof with a nail gun, and with the same nails used to build the roof trusses and the rest of the building frame. For many reasons workers are reluctant to carry too many tools. They are also reluctant, and it is inefficient, to have to exchange tools for different jobs. Ideally, a steel batten would be installable using a standard nail gun. However, due to the thickness of the material used to make steel battens, a nail gun will blow a nail right through a steel batten, and won't affix the batten to the truss below. This applies if the nail is inserted through the horizontal top of the top hat, or the flanges of the top hat. Additionally, traditional top hats also have narrow flanges which do not allow the application of a nail gun at an appropriate angle, limiting the ways in which the batten may be installed.
Typically, steel battens have been more expensive than timber battens due to the material used and the working required. Usually a steel batten is made from 0.55 mm steel. This provides a steel batten strong enough to hold the weight of any roofing material as well as providing a fall barrier when installed on the roof trusses. However, at current prices, a typical steel batten cannot compete cost-wise with wooden battens. Even the improvements in strength and worker safety do not make a standard steel batten competitive with a wooden batten.